The most frequent use of prostheses for skeleton reconstruction involves the total replacement of articulating joints which have been rendered nonfunctional, either by physical accident (trauma) or, more commonly, by one of several kinds of arthritis. The total prosthetic replacement of hip joints and knee joints is now widespread and there are also surgical procedures and prosthesis designs for the reconstruction of other joint systems, such as ankle, shoulder, finger, wrist and elbow joints.
The load bearing requirements of prostheses for skeletal reconstruction, especially in the lower limb, imposes special performance demands on the materials of construction. The joint replacement systems which have been most extensively used heretobefore involve a metal component, such as titanium and a polymer component which is coated onto one or both of the articulating surfaces of the joint. The polymer is preferably an ultrahigh molecular weight polyethylene which is both biocompatible and very resistant to wear. The parts of the prostheses which are implanted into the bone may be fixed in position with bone cement. More recently, however, it has been found that coating the parts of the prosthesis which are to be implanted into the bone with hydroxyapatite or a modified hydroxyapatite will host the growth of bone into the coating and thus assist in anchoring the implant in position. Hydroxyapatite is a member of the apatite group of minerals and has the chemical formula Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2. It is, essentially, a calcium phosphate including hydroxide having a Ca/P ratio of 1.67. Synthetic hydroxyapatite has the chemical structure given above and is similar structurally to bone material which thus makes it an ideal coating material for prostheses. Synthetic hydroxyapatite may be modified, for example, by substitution at the phosphate and hydroxyl sites, for example by carbonate groups.
The coating of implants with hydroxyapatite is described in a paper entitled "Contribution of Hydroxyapatite Coatings to Implant Fixation", by E. Munting, M. Verhelpen, Feng Li and A. Vincent from CRC Handbook of Bioactive Ceramics Volume II. Calcium Phosphate and Hydroxyapatite Ceramics, CRC Press Inc (1990).
The coating of the portions of the prostheses which are implanted into the bone with hydroxyapatite or a modified hydroxyapatite is carried out by well known techniques, for example by plasma spraying or sputtering. The plasma spray coating of hydroxyapatite is described in "Plasma Sprayed Coatings of Calcium Phosphate" by K. DeGroot, C. P. A. T. Klein, J. G. C. Wolke and J. M. A. de Bliek-Hagervorst from CRC Handbook of Bioactive Ceramics Volume II. Calcium Phosphate and Hydroxyapatite Ceramics, CRC Press Inc. (1990).
Low pressure plasma spraying of hydroxyapatite is generally preferred for the coating of prostheses. Using this technique the hydroxyapatite is injected into the plasma flame of a plasma gun where it melts. As the plasma steam impinges upon the substrate surface the hydroxyapatite in the plasma stream crystallizes and solidifies to form a hydroxyapatite coating.
Although for many applications titanium or other metal prostheses are satisfactory, they suffer from the disadvantage that they are relatively heavy, have a high Young's Modulus and require a surface roughening treatment prior to coating, such as by grit blasting, with the consequent difficulty of complete removal of debris following such a treatment. Accordingly, it has been suggested that prostheses should be based upon carbon fibers which are biocompatible, inert and strong. The carbon fibers require, however, to be bonded together into a composite of the desired shape for the prosthesis by using a synthetic polymer such as polybutyleneterephthalate. An acetabulum for a total hip prosthesis, which may be made from a carbon fiber reinforced plastics material is described in EP-A-0552949.
The plasma spraying of hydroxyapatite onto carbon fiber reinforced thermoplastic composite materials is disclosed in a paper entitled "Plasma-sprayed hydroxylapatite coating on carbon fiber reinforced thermoplastic composite materials", by S. -W. Ha, J. Mayer, B. Koch and E. Wintermantel, Journal of Materials Science, Materials in Medicine 5 (1994), 481-484. The materials used were carbon fiber reinforced PEEK prepared by hot pressing. These composites contained approximately 60 vol % fibers and were sandblasted and then coated with a layer of hydroxyapatite 200 .mu.m in thickness by plasma spraying. The authors of the paper report that carbon fibers in the outer layers of the composites were damaged in the plasma spraying process and that the adhesion between the plasma sprayed hydroxyapatite coating and the carbon fiber reinforced composite was very low.
Although a hydroxyapatite coating on a carbon fiber prosthesis would be desirable, the conventional plasma spraying techniques generally used for coating onto metals such as titanium cannot be used, because of the problems encountered as discussed above and, in particular, the degradation of the carbon fiber composite.